1. Field of the Invention
The present invention relates to a silicon nitride sintered body having excellent mechanical strengths especially at ordinary temperature and excellent in productivity and cost efficiency.
2. Description of the Prior Art
Various research and development have heretofore been made on the sintering process, sintering aid, restriction of constituent crystal phases and the like for the purpose of enhancing the strength of silicon nitride-based materials. For example, with regard to the sintering process, a sintered body having a bending strength as high as 100 kg mm.sup.2 has been embodied by the hot press sintering process (see Am. Ceram. Soc. Bull., 52 (1973), p.560), and also the hot isostatic press (HIP) process using a glass capsule has been developed. However, these processes cannot be thought to be excellent in productivity and cost efficiency, though excellent strength characteristics of a sintered body can be achieved by them. On the other hand, the gas pressure sintering process can cope with such a problem [see, for example, Mitomo, Funtai to Kogyo 21 (12), 27 (1989)]. However, since in the above process the densification of final sintered product is accompanied by the growth of .beta.-silicon nitride grain, thereby increasing the possibility of causing deterioration in strength due to coarse grain precipitation, and the sintering is carried out usually under a nitrogen gas pressure of 10 atm or higher, the process usually requires large sintering equipment as is the cases with the hot press process and HIP process, thus failing to render itself excellent in characteristics and productivity. Regarding sintering aids, there is disclosed an Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3 -based silicon nitride sintered body using Y.sub.2 O.sub.3 as the principal sintering aid in Japanese Patent Publication Nos. 49-21091 and 48-38448. As described in the specifications of the above published patents, it is believed that .beta.-silicon nitride grains form a fibrous structure in the sintered body and the structure is dispersed in the matrix, thus enhancing the strength and toughness of the sintered body itself. Specifically, in the aforestated sinter, the fact that .beta.-silicon nitride crystal is hexagonal and anisotropically grows in the direction of C-axis is positively utilized. As indicated in the Japanese Patent Publication No. 48-38448 and J. Ceramic Soc. Japan 94, p96 (1986 ), fibrous .beta.-silicon nitride grains sometimes grow by ten-odd .mu.m or larger in the C-axis direction. The above-mentioned sinter, however, still has a possibility that the grain growth causes the abnormal growth, the occurrence of pores or the deterioration of the strength of the sinter. Moreover, the above sintered body using the sintering aid only cannot be sufficiently densified unless the sintering temperature is raised to 1700.degree. to 1900.degree. C., and sometimes stable sintered products cannot be obtained owing to the sublimation and decomposition of silicon nitride in the case of the nitrogen gas pressure sintering around atmospheric pressure. Accordingly, the above sintered body cannot be said to be sufficiently excellent in the characteristics and productivity. According to any of the above-mentioned sintering processes, the strength of the obtained sintered body is approximately 100 kg/mm.sup.2 at the most expressed in terms of 3-point bending strength, for example, in accordance with JIS R-1601. Thus, considering a variety of applications of silicon nitride-based materials, sufficient characteristics of the materials have not been attained.